No. 12 of 19 Geosynthetics in Erosion Protection by Dr David Elton, P.E Auburn University

1. No. 12 of 19 Geosynthetics in Erosion Protection by Dr David Elton, P.E Auburn University The information presented in this document has been reviewed by the Education Committee of the International Geosynthetics Society and is believed to fairly represent the current state of practice. However, the International Geosynthetics Society does not accept any liability arising in any way from use of the information presented.

2. Geosynthetics in Sediment and Erosion Control • Introduction and Applications • What is it? • Erosion control is a means of keeping a soil in place or catching a soil after it has been displaced but before it moves into surface waters.

3. Why Is It Needed? There are public laws that : • preclude the polluting of surface waters with sediments • preserve topographic integrity • preserve soil for farming • preserve foundation integrity for structures founded on soil.

4. Rill and Gully Slope Erosion

5. Riverbank Erosion, Including Sapping (Formation of Caves)

6. Where Is It Needed? • It is needed on construction and agricultural sites and natural places where water causes soils to displace. • In short: where soil and moving water interact at the ground surface

7. Factors Influencing Types of Erosion • Rainfall-induced erosion factors: • intensity and duration of rainfall • slope of land • soil type • All affect the amount of erosion and the erosion control measures selected.

8. Factors Influencing Types Of Erosion (Continued) • Shoreline erosion factors: • soil type • wave height • beach slope • duration and intensity of storm • All affect the amount of erosion.

9. Factors Influencing Types Of Erosion (Continued) • Scour: • The amount of scour, around bridge piers for example, is affected by pier shape, depth of stream, storm duration and channel shape, in addition to soil type.

10. Hard Armor Erosion Control on Riverbank Concrete cast in a geotextile former; geotextile filter underneath(not visible)

11. Design Approach • Slopes and Channels Channel erosion damage caused by inadequate filter beneath hard armor

12. Strategy • Choose least costly erosion control measure and evaluate: • LOW cost nothing (fallow ground) plants degradable RECPs permanent RECPs permanent TRMs soft armor • HIGH cost hard armor w/geotextile filter • In conjunction with these choices, consider: • reducing flow in any manner • flattening slopes • widening channels

13. Design Procedures for Erosion Control in Slopes and Channels Slopes • There are several methods of estimating soil loss. The most commonly used in the US is: • USLE - Universal Soil Loss Equation: A = R  K  LS  C  P where: A = computed soil loss (tons/acre or kg/hectare) for a given storm period or time interval R = rainfall factor K = soil erodibility value LS = slope length and steepness factor C = vegetation or cover factor P = erosion control practice factor • All factors, except C, do not vary more than one order of magnitude. C changes several orders of magnitudes. • NOTE: many of these factors are described in USDA (1997)

14. Cover Index factor (C) for Different Ground Cover Conditions Type of Cover Factor Percent C Effectiveness None (fallow ground) 1.0 0.0 Temporary seedings (90% stand) Ryegrass (perennial type) 0.05 95 Ryegrass (annuals) 0.10 90 Small grain 0.05 95 Millet or sudan grass 0.05 95 Field bromegrass 0.03 97 Permanent seedings (90% stand) 0.01 99 Sod (laid immediately) 0.01 99 Mulch Hay, rate of application, tons/ac: 0.5 0.25 75 1.0 0.13 87 2.0 0.02 98 Small grain straw 2.0 0.02 98 Wood chips 6.0 0.06 94 Wood cellulose 1.5 0.10 90 Fiberglas 1.5 0.05 95 Source: primarily HEC-15 (1988) percent soil loss reduction as compared with fallow ground

15. Proposed C-Factors for RECPs Update of RUSLE Equation RECP Approximate Reported Proposed Category Mass/Unit Range of C-factors Area C-factors g/m2 (oz/ yd2) ECN 34 to 100 100% Woven 0.02 0.01 (1 to 3) Polypropylene 400 to 880 100% Woven 0.002 to 0.003 0.01 to 0.1 (12 to 26) Coir/Jute ECB 270 to 340 100% Straw 0.002 to 0.30 0.01 (8 to 10) 270 to 370 Straw/Coconut 0.002 to 0.11 0.01 (8 to 11) 270 to 400 100% Coconut/ 0.003 to 0.09 0.01 (8 to 12) Excelsior TRM 270 to 490 100% Synthetic 0.003 to 0.11 0.01 (8 to 14) NOTES/REFERENCES All terminology consistent with the Erosion Control Technology Council approved terms. “ECN” indicates temporary degradable erosion control net. Values assumes slope is flatter than 2H:1V. “ECB” indicates temporary degradable erosion control blanket. Values assumes slope is flatter than 2H:1V. “TRM” indicate permanent nondegradable turf reinforcement mat. Values assumes slope is flatter than 1H:1V.

16. Procedure • Calculate A with the C value of a given permanent erosion control solution (usually vegetation) • Compare A with an acceptable A (e.g. 44 kN/ha/year (2 tons/acre/year)) • If A is acceptable*, check effectiveness of the temporary (degradable) erosion control (RECP) measure used to establish the permanent erosion control solution over the life of the RECP. Choose a temporary solution with a small enough C to satisfy regulations. • *typically, acceptance is based on government regulations. If A is unacceptable for permanent solution (vegetation), try vegetation plus turf reinforcement mat for long term solution. C values available from test or manufacturers. • If that combination produces a satisfactory A, check A for temporary erosion control solution used while permanent solution is taking hold.

17. Channel Linings • Two Common Methods of Analysis: • Permissible velocity in channel • Permissible shear stress in channel

18. Velocity Calculation where: V - velocity of flow (ft/sec) n - Manning's roughness coefficient (see Table 3) R - hydraulic radius ( A / wetted perimeter) (ft) Sf - slope of channel, for uniform flow conditions.

19. In SI units, this equation becomes: V - velocity of flow (m3/sec) n - Manning's roughness coefficient (see Table 3) R - hydraulic radius ( A / wetted perimeter) (m) Sf - slope of channel, for uniform flow conditions. • Compare calculated V with an acceptable V from standard tables or manufacturer's literature

20. Table 3. Manning’s Roughness Coefficients. n - value1 Depth Ranges 0-0.5 ft 0.5-2.0 ft >2.0 ft Lining Lining type (0-15cm) (15-60 cm) (> 60 cm) Category Rigid Concrete 0.015 0.013 0.013 Grouted riprap 0.040 0.030 0.028 Stone masonry 0.042 0.032 0.030 Soil cement 0.025 0.022 0.020 Asphalt 0.018 0.016 0.016 Unlined Bare soil 0.023 0.020 0.020 Rock cut 0.045 0.035 0.025 Temporary Woven paper net 0.016 0.015 0.015 Jute net 0.028 0.022 0.019 Fiberglass roving 0.028 0.021 0.019 Straw with net 0.065 0.033 0.025 Curled wood mat 0.066 0.035 0.028 Synthetic mat 0.036 0.025 0.021 Gravel Riprap 1-inch (2.5-cm) D50 0.044 0.033 0.030 2-inch (5-cm) D50 0.066 0.041 0.034 Rock Riprap 6-inch (15-cm) D50 0.104 0.069 0.035 12-inch (30-cm) D50 -- 0.078 0.040 1Based on data primarily from HEC-15 (Chen and Cotton, 1988)

21. Notes: • Values listed are representative values for the respective depth ranges. • Manning’s roughness coefficients, n, vary with the flow depth. • n-values for vegetative linings are found in Chen and Cotton (1988) (HEC-15) on pages 42 - 46. • Another method is to evaluate the shear stress on the ground surface caused by the running water. (Method follows).

22. Shear Stress Calculation • maximum shear stress on channel base: where:  - shear stress w - unit weight of water d - depth of flow Sf - gradient of channel for uniform flow conditions • Calculate the expected shear stress and compare with acceptable shear stress

23. Design Variables • increasing the width/depth ratio of channel reduces  • channel slope: flatter reduces  (note: 10% is considered a "steep Channel". Sf > 10% usually requires hard armor)

24. Design Procedure for Sf < 10% Channels Shear stress approach : • calculate the maximum  • Note: this requires d. If Q is known, and the gradient and lining material are known, d can be found from Chart 3.

25. If Q is unknown, use Manning's equation to get Q where: Q = flow (cfs) n - Manning's roughness coefficient (see Table 3) A - cross-sectional area of channel (ft2) R - hydraulic radius ( A / wetted perimeter) (ft) Sf - slope of channel for uniform flow conditions. • Then, knowing Q, use Chart 3 to get d. With d, calculate maximum shear, , and compare with tabulated values of  for RECPs (Tables 1 and 2 from HEC-15) or with manufacturer's data.

26. In SI units, this equation is: where: Q = flow (m3/sec) n - Manning's roughness coefficient (see Table 3) A - cross-sectional area of channel (m2) R - hydraulic radius ( A / wetted perimeter) (m) Sf - slope of channel for uniform flow conditions.

27. Design Chart / Nomogram

28. Table 1. Classification of Vegetal Covers as to Degree of Retardance. Retardance Cover Condition Class A Weeping lovegrass Excellent stand, tall (average 30") (76 cm) Yellow bluestem Ischaemum Excellent stand, tall (average 36") (91 cm) Kudzu Very dense, growth, uncut Bermuda grass Good stand, tall (average 12") (30 cm) Native grass mixture. (little bluestem, bluestem, blue gamma, and other long B and short midwest grasses) Good stand, unmowed Weeping lovegrass Good stand, tall (average 24") (61 cm) Lespedeza sericea Good stand, not woody, tall (average 19") Alfalfa (48 cm) Weeping lovegrass Good stand, uncut (average 11") (28 cm) Kudzu Good stand, unmowed (average 13") (33 cm) Blue gamma Dense growth, uncut Good stand, uncut (average 13") (28 cm)

29. Table 1. Classification of Vegetal Covers as to Degree of Retardance. Retardance Cover Condition Class Crabgrass Fair stand, uncut (25 to 120 cm) Bermuda grass Good stand, mowed (average 15 cm) Common lespedeza Good stand, uncut (average 28 cm) Grass-legume mixture-- C summer (orchard grass, redtop, Italian ryegrass, and common lespedeza Good stand, uncut (15 to 20cm) Centipedegrass Kentucky bluegrass Very dense cover (average 15 cm) Bermuda grass Good stand, headed 15 to 30cm Common Lespedeza Good stand, cut to 6 cm Buffalo grass Excellent stand, uncut (11 cm) Grass-legume mixture Good stand, uncut (8 to 15 cm) fall, spring (orchard grass, Good stand, uncut (10 to 13cm) D redtop, Italian ryegrass, After cutting to (5 cm) Very good and common lespedeza) stand before cutting Lespedeza sericea

30. Table 1. Classification of Vegetal Covers as to Degree of Retardance. (HEC - 15) Retardance Cover Condition Class E Bermuda grass Good stand, cut 4cm height Bermuda grass Burned stubble Note: Covers classified have been tested in experimental channels. Covers were green and generally uniform.

31. Table 2. Permissible Shear Stresses for Lining Materials Permissible Unit Shear Stress1 Lining Category Lining Type (lb/ft2) (kg/m2) Temporary Woven Paper Net 0.15 0.73 Jute Net 0.45 2.20 Fiberglass Roving: Single 0.60 2.93 Double 0.85 4.15 Straw with Net 1.45 7.08 Curled Wood Mat 1.55 7.57 Synthetic Mat 2.00 9.76 Vegetative Class A 3.70 18.06 Class B 2.10 10.25 Class C 1.00 4.88 Class D 0.60 2.93 Class E 0.35 1.71 Gravel Riprap 1-inch (2.54 cm) 0.33 1.61 2-inch (5 cm) 0.67 3.22

32. Table 2. Permissible Shear Stresses for Lining Materials Permissible Unit Shear Stress1 Lining Category Lining Type (lb/ft2) (kg/m2) Rock Riprap 6-inch (15 cm) 2.00 9.76 12-inch (30 cm) 4.00 19.52 Bare Soil Non-cohesive See Chart 1 Cohesive See Chart 2 1ref: HEC - 15 For fallow cohesionless soils, compare with Chart 1 note: if particle diameters are larger than 100mm (0.33 ft), use  = 25.5 D50 with D50 being the mean rock size in meters, and  in kPa, or  = 4 D50 with D50 being the mean rock size in feet, and  in psf.

34. Ref. Chen and Cotton (1988)

36. Erosion Control Using Geosynthetics Applications: • Introduction • Useful in scour, surface and shoreline protection • Geosynthetic functions include: • filtration • containment (bags) • protection • providing a medium for plant growth

37. Erosion Control Using Geosynthetics Scour Applications • Bridge pier footing • Canal Lining

38. Surface Protection Philosophy • reduce the intensity of the raindrops impacting the soil, reduce the speed of runoff, increase the amount of water that soaks into the soil rather than running off. Roving • Roving is fine threads spread out on the ground surface, tacked down with a spray that holds it in place while vegetation takes hold. • Roving is applied manually, with a light machine. The method is slow, but useful in smaller areas with uneven surfaces.

39. spools of thread • placed using air guns Roving Materials

40. Roving Being Applied • Roving (white) being sprayed on to the ground surface, • Being tacked down with asphalt spray

41. Rolled Erosion Control Product Being installed in a ditch

42. Permanent Installations (Permanent Erosion and Revegetation Mats) Soft PERMs • Turf Reinforcement Mats (TRMs) • TRMs are typically placed on the surface and then filled in with soil. They reinforce the ground surface, making erosion more difficult. The TERM holds the soil in place while vegetation takes hold. • Erosion Control and Revegetation Mats (ECRMs) • These combine surface control and surface slope stabilization at the same time.

43. Geocell Confinement Systems (GCS) Geocellular confinement system on a slope being filled with soil

44. GCS – Soil Fill GCS are an expensive, rugged way to stabilize a slope or roadbed. These cellular mats are filled with soil. They are very strong and very effective.

45. Hard Permeable System • Gabions - wire baskets filled with cobble-sized rocks • Hard armor: Gabion channel lining (geotextile filter underneath not visible) • Loose stone – riprap. Random placement is best. There are various methods for estimating how large these rocks must be to avoid displacement.

46. Hard Armor • Riprap being installed with geotextile filter underneath • Concrete or masonry: Dolos (large concrete objects), articulated blocks (forming a mat), concrete facings (cast in place or precast)

47. Geotextile filter underneath not visible Hard Armor : Dolos

48. Hard Armor Articulated blocks with geotextile filter underneath

49. Design Aspects Wave action - provides dynamic loads on filters - installation procedures include careful compaction and inspection - “Structural numbers” used to determine depth of hard armor protection (John 1987) Structural Numbers (SN) for use in erosion control Hard Armor Type Required Structural Number unbonded riprap < 2 (worst case) free blocks < 2 asphalt grouted open aggregate < 4.3 sand filled mattresses < 5 articulated blocks < 5.7 grouted articulated blocks < 8

50. Design Aspects • For severe cases, use Hudson's formula to calculate required stone size instead of depth of stone. where: W = weight of stone needed H = wave height  = slope angle D = 2.73 (typical) Gs = specific gravity of stone s = unit weight of stone